Jessie, a nine-year-old Jack Russell, had to undergo surgery after the growth caused her abdomen to expand.

Shocked owner Mandy Cracknell began to worry when her beloved pet grew bigger and bigger in the space of just a fortnight. She could not understand what was causing her dog's rapid weight gain, so took Jessie to the Leicester PetAid Hospital run by veterinary charity the PDSA.

Jessie had an ultrasound scan and X-rays which revealed a large mass in her abdomen. The vets investigated further and found a non-cancerous tumour weighing 1.6kg, which was more than 20% of the dog's entire bodyweight.

Chris Sherwood, PDSA veterinary surgeon, carried out an operation and discovered that the growth was on one of Jessie's ovaries. He said he had removed non-cancerous tumours the size of basketballs in larger breeds, but was amazed to see one so big in such a small dog.

"Jessie only weighed 7.8kg before the surgery so you can appreciate how big this mass was," he said. "Something the size of a melon in a dog as small as Jessie is remarkable and we removed a mass that was something like a fifth of her bodyweight. We also had to neuter Jessie to make sure this condition didn't happen again."

Mrs Cracknell, 44, from Leicester, said Jessie has gone on to make a full recovery following the removal of the tumour in April.

"I couldn't believe it when Jessie started putting on the weight," she said. "She seemed fine in herself as she was still eating and acting normally. It was such a shock to find out it was such a huge mass and when they told me it was the size of a melon, I couldn't believe it.

"I'm so grateful to PDSA for the treatment and care they gave Jessie and she's now back to her lovely, active old self."

In some cases the tumours - which can be found in cats, dogs and rabbits - are cancerous and can spread to other internal organs, which can prove fatal. The PDSA recommends getting pets neutered, especially rabbits, which have an 85% chance of developing cancer of the womb or ovaries by the age of three.